This invention relates to hydraulic machines such as Francis type pump-turbines, centrifugal pumps and the like, and more particularly of a type in which the strength for resisting vibration of the runner can be substantially improved, so that high-speed and high pressure-head operation of the machine is made possible.
An ordinary Francis type hydraulic turbine generally comprises a turbine shaft and a runner secured to one end of the turbine shaft, the runner being encased in a runner chamber which is defined between an upper cover and a lower cover. A plurality of guide vanes are provided around the runner chamber on the outside thereof, and a spiral casing is provided on the outside of the guide vanes. When the turbine is to be operated, water in the spiral casing is supplied through the guide vanes into the runner chamber and the reaction force created at this time is utilized for rotating the runner.
In this case, it has been known that each time when a runner blade passes the proximity of the guide vanes which are held stationary, there is a tendency of creating pulsation in water pressure, caused by mutual actions between these members. The pulsation in water pressure is transmitted to the runner surface to vibrate the same in the vertical direction.
In conventional hydraulic turbines which are operated under comparatively low heads and hence at comparatively low speeds, the pulsation in water pressure has a comparatively low energy, so that the influence thereof on the vibration resisting strength of the runner is negligible. However, in a recent hydraulic turbine operating under a high water head and at a high rotating speed, the energy of the water pulsation tends to increase such that the effects thereof on the vibration resisting strength of the runner can not be negligible. Particularly, when the pulsation frequency coincides with the natural vibration frequency of the runner, the operation of the hydraulic turbine becomes dangerous.